Created files for AMOEBA 2013 force field
Added documentation and header information for AMOEBA 2013 force field
AmoebaTorsionTorsionForce can calculate derivatives of the grid points automatically
Fixed incorrect specification of torsion-torsion terms in AMOEBA 2013 force field
Regenerated AMOEBA 2013 force field based on corrected parameter file
Merge branch 'amoeba' of https://github.com/peastman/openmm into peastman-amoeba
Merge branch 'peastman-amoeba'
Merge pull request #455 from peastman/master

Implemented AMOEBA 2013 force field